August 27, 2014

Genetic Modification, Invasive Species Overlooked In Calculation Of Biomass Production Limits

Recent increases in human population and economic growth have increased the demand for land-plant biomass for food, fuel and other purposes, but according to scientists, the supply of the sum of leaf, stem, root, fruit and other terrestrial plant-based materials has been hampered by a limit to what can naturally be produced.

However, in new research appearing in a recent edition of the journal Environmental Science and Technology, University of Illinois plant biology professor Evan DeLucia and his colleagues reveal that this so-called theoretical limit of terrestrial plant productivity is actually much higher than previously believed.

“When you try to estimate something over the whole planet, you have to make some simplifying assumptions,” DeLucia said in a statement. “And most previous research assumes that the maximum productivity you could get out of a landscape is what the natural ecosystem would have produced. But it turns out that in nature very few plants have evolved to maximize their growth rates.”

DeLucia and a team of experts from the University of Illinois, Colorado State University and the USDA Agricultural Research Service’s Photosynthesis Research Unit explained that, based on estimates derived from satellite images of vegetation and modeling, roughly "54 gigatons of carbon is converted into terrestrial plant biomass each year."

Over the past several decades, that biomass value has remained stable, which has led many scientists to conclude that it represents an upper limit on the amount that can be produced worldwide. However, the authors of the new study suggest that these assumptions overlook key factors such as human attempts that could increase overall plant productivity – efforts such as genetic manipulation, plant breeding and land management.

According to DeLucia, some work in this field has already enjoyed great success. For instance, Miscanthus x giganteus, a hybrid grass produced in Illinois without the use of fertilizers or irrigation, produced between 10 and 16 tons of above-ground biomass per acre – more than doubling the output of native prairie vegetation or corn. In addition, genetically modified no-till corn produces five-times more total biomass per acre than restored prairie.

Overall biomass production could also be increased by introducing invasive or non-native species to new areas. While DeLucia and his colleagues caution this could be harmful to some ecosystems, non-native species nonetheless demonstrate the potential for increasing the overall productivity levels of plants.

For example, a non-native species introduced in Iceland (the nootka lupine) produces four times more biomass than the species it displaces (the boreal dwarf birch), while Indian bamboo plantations produce roughly 40 percent more biomass than native dry, deciduous tropical forests. These examples prove that the net primary production (NPP) of plants has yet not been maxed out, the study authors explain in their paper.

The research team used what is known as a simple light-use efficiency model in combination with the theoretical maximum efficiency with which solar radiation is converted to biomass by plant canopies to estimate the theoretical NPP limit on a worldwide scale. Their newly calculated limit was approximately “two orders of magnitude higher” than the biomass productivity levels of most current managed or natural ecosystems.

“We're not saying that this is even approachable, but the theory tells us that what is possible on the planet is much, much higher than what current estimates are,” DeLucia explained. “Taking into account global water limitations reduced this theoretical limit by more than 20 percent in all parts of the terrestrial landscape except the tropics, but even that water-limited NPP is many times higher than we see in our current agricultural systems.”

Image 2 (below): Scientists have historically underestimated the potential productivity of the earth's land plants, researchers report in a new study. Credit: NASA Earth Observatory image by Jesse Allen